One keepers made a mistake, the other has possibly not (I can’t remember all of the shots he’s faced). I’m really not sure what all of this proves so early in the season. You keep referring to a terrible WC for De Gea but you could also say the same for most of that Spanish squad. And the thing about he isn’t playing as well as he used... well it’s 4 games in. Come back in 6 months and discuss whether De Gea is past his best.
Also the save rate percentage isn’t the be all and end all. Another stat which in some cases is useless.